1. Field of the Invention
The present invention relates to an image reading apparatus such as an image scanner used by being connected to an external apparatus such as a personal computer or a scanner part provided in a copier or a facsimile machine, and to a copier including such an image reading apparatus.
2. Description of the Related Art
An image scanner including a buffer memory for storing read image data is connected to a PC or a network of lines to which the image data stored in the buffer memory is output. In this case, depending on the data processing capability of the PC or the network of lines or on a data transfer rate at which the image data is output from the buffer memory, the image scanning capability of the image scanner may not be balanced with the data transfer rate. Therefore, the conventional image scanner stops scanning for image reading and waits for the image data in the buffer memory to be transferred to the PC or the network of lines if the buffer memory is filled to its storage capacity.
A great light amount variation (a variation in an amount of light to an elapsed time) is caused in a light source for scanning for image reading during a period immediately after the light source is turned on until the light source is stabilized so as to affect the lightness of an image even if a highly stable Xe lamp is employed as the light source. Further, in the case of a color image, if a white fluorescent lamp is employed as the light source, differences are caused among amounts of light of colors of R (red), G (green), and B (blue) even though the same period of time passes. Therefore, a color difference is caused between images on the leading and trailing edges of one frame.
Therefore, if the above-described stoppage of the scanning for image reading lasts for more than a few seconds due to the fact that the buffer memory is filled to its storage capacity, the effect of the light amount variation in the light source for exposure scanning becomes more apparent in the image (in terms of lightness or color difference). A change in the image is great and extremely noticeable particularly at a joint of partial images read before and after the stoppage of the scanning for image reading. This results in a problem of deterioration of image quality.
In order to solve such a problem, correction of a variation in a read image caused by a variation with time in the amount of light of a light source (hereinafter referred to as light amount variation correction) is performed, as disclosed in Japanese Laid-Open Patent Application No. 10-308849. According to a technique disclosed therein, a reference reflection member is provided in a sub scanning direction to intercept a stray light included in a reflected light from an original document so that light amount correction is properly performed. Further, since a light source characteristically has its illuminance increased or decreased monotonously from a read start side to a read end side, a read correction error is reduced even if a displacement or shift occurs in an optical system.
If the light source continues to be on after the power switch of the image scanner is turned on, the amount of light is stabilized in a few minutes thereafter. Therefore, from that moment, only a small variation is caused in the amount of light. In this case, however, the light source is kept on for a long period of time in other operations than a scanning operation for image reading. This inappropriately causes a-problem of a shortened light source life.
However, according to the technique disclosed in Japanese Laid-Open Patent Application No. 10-308849, since a light reflected from the reference reflection member is detected in different positions in the sub scanning direction every time, a strict level of reflection uniformity in the sub scanning direction is required of the reference reflection member. Therefore, it is necessary to paint or apply a decal to the base material, which is a metal or resin, of the reference reflection member so as to achieve uniformity of the reflection surface thereof, thus resulting in an increase in costs.
Further, since a strict level of reflection uniformity in the sub scanning direction is required of the reference reflection member, the light amount correction is not normally performed at the time of reading the reflected light using a position on the reference reflection member if the position is dirty.
Furthermore, since the light amount correction, which is performed in a short period of time in an interval during motor control or image processing control, applies a large amount of load to a central processing unit (CPU) in control of each part. Therefore, depending on the processing capability of the CPU, another problem is caused that image processing capability should be decreased by reducing a rate of scanning for image reading so as to reduce the load on the control of the CPU.